Substrate-free 2D tellurene

ABSTRACT

The present disclosure generally relates to compositions comprising substrate-free 2D tellurene crystals, and the method of making and using the substrate-free 2D tellurene crystals. The 2D tellurene crystals of the present disclosure are characterized by an X-ray diffraction pattern (CuKα radiation, λ=1.54056 A) comprising a peak at 23.79 (2θ±0.1°) and optionally one or more peaks selected from the group consisting of 41.26, 47.79, 50.41, and 64.43 (2θ±0.1°).

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefits of U.S. Provisional ApplicationSer. No. 62/521,695, filed Jun. 19, 2017. The contents of which areincorporated herein entirely

STATEMENT REGARDING PRIOR DISCLOSURES BY THE INVENTORS OR JOINTINVENTORS UNDER 37 C.F.R. 1.77(b)(6)

Wenzhuo Wu and Yixiu Wang, the inventors or joint inventors of thepresent disclosure, publicly disclosed information related to thepresent disclosure in article Wang, Yixiu, et al., Large-areasolution-grown 2D tellurene for air-stable, high-performancefield-effect transistors, arXiv:1704.06202, Apr. 20, 2017. The articlewas first published online on Apr. 20, 2017, which is less than one yearfrom the filing date of the U.S. Provisional Application Ser. No.521,695, filed Jun. 19, 2017. The other eight listed co-authors GangQiu, Qingxiao Wang, Yuanyue Liu, Yuchen Du, Ruoxing Wang, William A.Goddard III, Moon J. Kim, and Peide D. Ye of the article are notinventors for the present disclosure because the eight listed co-authorsonly provided supervised contributions instead of providing inventivecontribution. A copy of a print out of the article is provided on aconcurrently filed Information Disclosure Statement (IDS).

TECHNICAL FIELD

The present disclosure generally relates to compositions comprisingsubstrate-free 2D tellurene crystals, and the method of making and usingthe substrate-free 2D tellurene crystals.

BACKGROUND

This section introduces aspects that may help facilitate a betterunderstanding of the disclosure. Accordingly, these statements are to beread in this light and are not to be understood as admissions about whatis or is not prior art.

Research in 2D materials, as inspired by the development of graphene,has experienced an explosive increase in recent years, due to theirunique and exceptional properties with promising applications inelectronic, photonic, energy and environmental devices. The 2D group-IVmaterials including silicene, germanene and stanene have been realizedexperimentally after graphene. For group-V elements, few-layer blackphosphorus, named phosphorene, has also been successfully fabricated byexfoliation, which exhibits prominent properties such as high carriermobility and high on/off ratio. Very recently, the novel 2D group-IIImaterial of borophene has been fabricated successfully. Beside theallotropes of single element in 2D family, the 2D transition metaldichalcogenides, such as MoS₂, MoSe₂, WS₂, and WSe₂, have beensynthesized and attracted both experimental and theoretical interestsbecause of their relatively large and direct band gap as well as goodcarrier mobilities.

The 2D structures of simple group VI elements has only recently beenpredicted to be possible. See Zhu et al., Tellurene-a monolayer oftellurium from first-principles prediction, doi:arXiv:1605.03253 (2016).In addition, Chen et al. reported 2D tellurene grown on highly orientedpyrolytic graphite (HOPG) substrate by molecular-beam epitaxy (MBE). SeeChen et al., Ultrathin layers of beta-tellurene grown on highly orientedpyrolytic graphite by molecular-beam epitaxy, arXiv:1704.07529 (2017).

There remains a need to develop a method to make substrate-free 2Dtellurene due to its potential applications in electronics,optoelectronics, energy conversion and energy storage.

SUMMARY

The present disclosure provides substrate-free 2D tellurene crystalsthat can be reliably prepared through a substrate-free solution-grownmethod.

The crystals exhibit process-tunable thicknesses from a few to tens ofnanometers, and lateral sizes up to 500 μm. The tellurene transistorspresent air-stable performance at room temperature for over two months,with on/off ratios and current densities on the order of 10⁶ and 550mA/mm, respectively. The thickness-dependent carrier mobility reachesthe highest values ˜700 cm²/Vs for a thickness of ˜16 nm. The tellurenealso shows strong in-plane anisotropic properties. The novel versatilesolution-grown process allows the access to a broad range ofcharacterization and application of tellurene as a new 2D material forelectronic, optical optoelectronic, sensing, and energy devices.

In one embodiment, the present disclosure provides substrate-free 2Dtellurene crystals.

In one embodiment, the present disclosure provides substrate-free 2Dtellurene crystals wherein the 2D tellurene crystals have a laterallength from 10-500 μm.

In one embodiment, the present disclosure provides a substrate-free 2Dtellurene crystals wherein the 2D tellurene crystals have a thicknessfrom 0.5-500 nm.

In one embodiment, the present disclosure provides a method of preparingsubstrate-free 2D tellurene crystals, wherein the method comprisesreacting sodium tellurite (Na₂TeO₃) and hydrazine (N₂H₄) in an alkalinecondition (pH>7) with the presence of polyvinylpyrrolidone (PVP) toprovide 2D tellurene, adding acetone to the as-synthesized 2D tellurenesolution to provide 2D tellurene crystals with thickness between 0.5-10nm.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of thepresent disclosure, reference will now be made to the embodimentsillustrated in the drawings, and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of this disclosure is thereby intended.

In the present disclosure the term “about” can allow for a degree ofvariability in a value or range, for example, within 10%, within 5%, orwithin 1% of a stated value or of a stated limit of a range.

In the present disclosure, the term “2D tellurene” refers to anallotrope of tellurium element. The term “2D tellurene” refers theallotrope in the form of two-dimensional, atomic scale, and hexagonallattice in which one atom forms each vertex. One well-known 2D materialis graphene, which is an allotrope of carbon.

In the present disclosure, the term “lateral” refers to either thelength or width of the 2D tellurene crystals. Therefore, “laterallength” may refer to either the length of the length side or width sideof the crystals.

In the present disclosure, the term “substrate-free 2D tellurene” refersto 2D tellurium crystals that are prepared through a solution conditioninstead of being deposited on a substrate as disclosed by Chen et al.regarding 2D tellurene grown on highly oriented pyrolytic graphite(HOPG) substrate by molecular-beam epitaxy (MBE). The disadvantage of 2Dtellurene grown on highly oriented pyrolytic graphite (HOPG) substrateis that such material cannot be easily used and is not available asstandalone 2D tellurene crystal. Therefore, the substrate-free 2Dtellurene disclosed in the present disclosure provides a standalone,stable and convenient source of pure 2D tellurene. A skilled artisanwill appreciate that “substrate-free 2D tellurene” refers to“substrate-free 2D tellurene” as made. The later prepared composition byany other physical and/or chemical mixing or combining the as made“substrate-free 2D tellurene” with another material, even could be namedas “substrate”, should still be within the definition of “substrate-free2D tellurene” as defined here.

Group VI tellurium (Te) has a trigonal crystal lattice in whichindividual helical chains of Te atoms are stacked together by van derWaals type bonds and spiral around axes parallel to the

direction at the center and corners of the hexagonal elementary cell.Each tellurium atom is covalently bonded with its two nearest neighborson the same chain. Tellurium can be considered as a 1D van der Waalssolid. Earlier studies revealed bulk Te has small effective masses andhigh hole mobilities due to spin-orbit coupling. The lone-pair andanti-bonding orbitals give rise to a slightly indirect bandgap in theinfrared regime (˜0.35 eV) in bulk Te. The conduction band minimum (CBM)in bulk Te is located at the H-point, while valence band maximum (VBM)is slightly shifted from the H-point along the chain direction, givingrise to hole pockets near H-point. When the thickness is reduced, theindirect feature becomes more prominent. For example, the VBM of 4-layerTe is further shifted to (0.43, 0.34) (in the unit of the surfacereciprocal cell), while CBM remains at (½, ⅓). Accompanied by the shiftof VBM, the band gap also increases due to the quantum confinementeffect, and eventually reaches ˜1 eV for monolayer Te20. Te has otherappealing properties, e.g. photoconductivity, thermoelectricity, andpiezoelectricity for applications in sensors, optoelectronics, andenergy devices. A wealth of synthetic methods has been developed toderive Te nanostructures, which favor the 1D form due to the inherentstructural anisotropy in Te. Much less is known about the 2D form of Teand related properties.

Large-area, high-quality, and substrate-free 2D Te crystals (termedtellurene) were synthesize with a solution process. The term X-ene isused to generally describe 2D forms of elemental materials withoutconsidering the specific bonding. The samples are grown through thereduction of sodium tellurite (Na₂TeO₃) by hydrazine hydrate (N₂H₄) inan alkaline solution at temperatures from 160-200° C., with the presenceof crystal-face-blocking ligand polyvinylpyrrolidone (PVP). Thetellurene flakes can be transferred and assembled at large scale,through a Langmuir-Blodgett process, onto various substrates forcharacterization and device integration. The structure, composition, andquality of these tellurene crystals have been analyzed by high angleannular dark field scanning transmission electron microscopy(HAADF-STEM), high-resolution transmission electron microscopy (HRTEM),energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD)No point defects or dislocations were observed over a large area withinsingle crystals. EDS result confirmed the chemical composition of Te.Similar characterizations and analyses of dozens of tellurene flakeswith different sizes indicate that all samples grow laterally along the<0001> and <1210> directions, with the vertical stacking along the<1010> directions.

The controlled PVP concentration is critical for obtaining 2D tellurene.When a smaller amount of PVP is used, the first 2D structures occurafter a shorter reaction time. A closer examination of reactions withdifferent PVP concentrations reveals an intriguing morphology evolutionin growth products with time. For each PVP concentration, the initialgrowth products are dominantly 1D nanostructures. After a certain periodof reaction, structures possessing both 1D and 2D characteristics startto emerge. TEM characterizations indicate that the long axes of theseflakes are <0001> oriented, and the lateral protruding regions) growalong the <1210> directions, with the {1010} facets as the top/bottomsurfaces. The 2D regions are enclosed by edges with atomic level steproughness. These high energy edges are not specific to certain planesduring the intermediate states. These structures also have more unevensurfaces compared to 2D tellurene, further manifesting theirintermediate nature. Finally, the ratio of 2D tellurene flakes whichhave a straight {1210} edge increases with a reduction in 1D andintermediate structures and reaches a plateau after an extended growth,e.g. ˜30 hours. The growth with a lower level of PVP has a smaller finalproductivity. The observed morphology evolution suggests that thebalance between the kinetic and thermodynamic growth dictates thetransformation from 1D structures to 2D forms. In the initial growth,PVP is preferentially adsorbed on the {1010} surfaces of the nucleatedseeds25, which promotes the kinetic-driven 1D growth. When the reactioncontinues, {1010} surfaces of the formed structures would becomepartially covered due to the insufficient PVP capping. Since {1010}surfaces have the lowest free energy in tellurium, the growth of {1010}surfaces along the <1210> direction significantly increases through thethermodynamic-driven assembly, giving rise to the observed intermediatestructures. The enhanced growth along the <1210> directions togetherwith the continued <0001> growth leads to the formation of 2D tellurene.

The sizes and thicknesses of tellurene can also be effectively modulatedby controlling the ratio between sodium tellurite and PVP. The width oftellurene monotonically decreases with the reduction of PVP level; thethickness is minimized when a medium level of PVP is used (e.g.Na2TeO3/PVP ratio ˜52.4/1), and increases with both the increase anddecrease of PVP. With a small amount of PVP, the solution issupersaturated with Te source, and homogeneous nucleation of Te canoccur in large scale, consuming resource for subsequent growth. As aresult, the Ostwald ripening of Te nuclei is shortened, and the finaltellurene crystals have smaller sizes compared to samples grown athigher PVP concentrations. The low PVP level also leads to moresignificant growth along thickness directions. On the other hand, whenthe PVP level is high, the fewer nucleation events allow the sufficientsupply of Te source for subsequent growth, leading to the increasedwidth and thickness. Also, the productivity of tellurene increases withthe reaction temperature from 160° C. to 180° C. This is likely becausehigher temperature promotes the forward reaction rate in the halfreaction of endothermic hydrazine oxidation. However, when temperatureincreases from 180° C. to 200° C., the possible breaking of the van derWaals bonds between Te chains by the excessive energy could lead to thesaturated productivity.

The 2D tellurene crystals with a thickness smaller than 10 nm can befurther derived through a solvent-assisted post-growth thinning process.The thickness of tellurene decreases with time after acetone isintroduced into the growth solution. After 6 hours, the averagethickness of tellurene is reduced to ˜10 nm, with the thinnest flakedown to 0.9 nm thick when sufficient amount of NaOH solution is addedinstead of adding acetone. Due to the poor solubility in acetone, PVPmolecules tend to desorb from the tellurene and undergo aggregation,giving rise to the sediment of tellurene over the time in acetone.Lacking the protection of PVP, the tellurene surfaces get exposed andreact with the alkaline growth solution (pH˜11.5), leading to thereduced thickness. Other types of solvents may provide similar effect.PVP solubility in the solvent may significantly affect the post-growththinning process.

These high-quality ultrathin tellurene crystals with controlledthicknesses provide an ideal system to explore their intrinsicproperties in the 2D limit. Back-gate tellurene transistors wasfabricated to assess their electronic properties. Excellent andair-stable transistor performances were achieved at room temperature forover two months without encapsulation. In particular, important metricsof devices such as on/off ratio, mobility, and current density aresuperior or comparable to transistors based on other 2D material. Sourceand drain regions were patterned by electron beam lithography with thechannel parallel to the [0001] direction of tellurene. By simply scalingdown the channel length to 100 nm, the maximum on current exceeds 550mA/mm, which stands out among most 2D materials based transistors. Suchlarge Ion values show the potential of the solution-grown tellurene inenabling novel low-power transistors. Remarkably, the tellurenetransistors demonstrate good stability in the air without encapsulation.No significant degradation was observed in the transfer curves for the15 nm tellurene transistor after two months being stored in air. Such anair-stability, superior to other materials such as phosphorene andsilicene9, is likely due to the weak binding of oxygen atoms to Tesurface, as shown by our first-principles calculations.

The process-tunable thickness of tellurene allows the modulation ofdevice performance in tellurene transistors through tuning theelectronic structures. We further explore the thickness dependence oftwo key metrics of material performance, on/off ratios and field-effectmobilities, to elucidate the transport mechanism of tellurene FETs.Field-effect mobilities with various thicknesses are displayed in FIG.3b. It peaks with ˜700 cm²/Vs at room temperature at around 16 nmthickness and decreases gradually with the further increase of thethickness. This trend is similar to layered materials that experiencescreening and interlayer coupling13, 14 (Supplementary Notes andSupplementary FIG. 15). We expect to be able to improve the mobility oftellurene through approaches such as surface passivation by high-kdielectric39 or h-BN. The thickness-dependent on/off ratios are shown inFIG. 3c, which steeply decrease from ˜10⁶ for a 4 nm crystal to lessthan 10 once the crystal thickness exceeds 25 nm, with a trend similarto reported layered materials13,14. The above results also indicate theimportance of thickness engineering in tellurene transistors fordifferent applications where the trade-off between higher mobility andbetter switching behavior need to be carefully considered, whichwarrants further investigations for testing the performance limit oftellurene FETs.

The in-plane anisotropy of few-layer tellurene flakes was also examined.Reduced in-plane symmetry in the crystal structure can lead tointeresting anisotropic properties that may enable novel functionalitiesand applications of 2D materials. The anisotropic optical properties ofas-synthesized tellurene was characterized by angle-resolvedpolarization Raman spectroscopy at room temperature. The incident lightcomes in along the [1010] direction and is polarized into the [0001]helical chain direction of the tellurene. Three Raman active modeslocating at 92 cm⁻¹ (E-mode), 121 cm-1 (Al mode) and 141 cm-1 (E-mode)were identified. By rotating the tellurene flake in steps of 30°,changes were observed in the angle-resolved Raman peak intensities. Thepeak intensities of different modes were extracted by fitting with Gaussfunction and plotted them into the corresponding polar figures. Theangle-resolved Raman results confirm that the helical Te atom chains inthe as-synthesized tellurene are oriented along the growth direction ofthe tellurene flake. The first-principle calculations show a similardegree of anisotropy in the effective masses along these two orthogonaldirections.

In summary, a simple, low-cost, solution-based chemical pathway to thescalable synthesis and assembly of 2D tellurene crystals was developed.These high-quality 2D ultrathin tellurene crystals have high carriermobility and are air-stable. This approach has the potential to producestable, high-quality, ultrathin semiconductors with a good control ofcomposition, structure, and dimensions for applications in electronics,optoelectronics, energy conversion, energy storage, sensors, and quantumdevices. Tellurene, as a 1D van der Waals solid, adds a new class ofnanomaterials to the large family of 2D crystals and enablespossibilities for the further investigation of many exciting propertiesand intriguing applications.

In one embodiment, the present disclosure provides substrate-free 2Dtellurene crystals.

In one embodiment, the present disclosure provides a compositioncomprising substrate-free 2D tellurene crystals.

In one embodiment, the present disclosure provides a compositioncomprising substrate-free 2D tellurene crystals, wherein the 2Dtellurene crystals comprising single crystals.

In one embodiment, the 2D tellurene crystals in the present disclosurehave a lateral length from 0.1-500 μm, 0.1-250 μm, 0.1-100 μm, 0.1-50μm, 10-500 μm, 10-250 μm, 10-100 μm, 10-50 μm, 20-500 μm, 20-250 μm,20-100 μm, 20-50 μm, 30-500 μm, 30-250 μm, 30-100 μm, 30-50 μm, 40-500μm, 40-250 μm, 40-100 μm, 40-50 μm.

In one embodiment, the substrate-free 2D tellurene crystals in thepresent disclosure have a thickness from about 0.5-250 nm, 0.5-200 nm,0.5-150 nm, 0.5-100 nm, 0.5-50 nm, 1-250 nm, 1-200 nm, 1-150 nm, 1-100nm, 1-50 nm.

In one embodiment, the substrate-free 2D tellurene crystals in thepresent disclosure are mono layer crystals. In one aspect, thesubstrate-free 2D tellurene crystals are at least two-layer crystals. Inone aspect, the substrate-free 2D tellurene crystals are 1-500 layercrystals. In one aspect, the substrate-free 2D tellurene crystals are1-250 layer crystals, 1-100 layer crystals, 1-50 layer crystals

In one embodiment, the present disclosure provides a method of preparingsubstrate-free 2D tellurene crystals, wherein the method comprisesreacting sodium tellurite (Na₂TeO₃) and hydrazine (N₂H₄) in an alkalinecondition (pH>7) with the presence of polyvinylpyrrolidone (PVP).

In one embodiment, the present disclosure provides a method of preparingsubstrate-free 2D tellurene crystals, wherein the method comprisesreacting sodium tellurite (Na₂TeO₃) and hydrazine (N₂H₄) in an alkalinecondition (pH>7) with the presence of polyvinylpyrrolidone (PVP),wherein the mole ratio of Na₂TeO₃/PVP is at least 5:1. In one aspect,the Na₂TeO₃/PVP ratio is at least 10:1, 15:1, 20:1, 30:1, 40:1, 50:1,60:1, 70:1, 80:1, 90:1, 100:1, 200:1, 300:1, 400:1, 500:1. In oneaspect, the Na₂TeO₃/PVP ratio is about 5-500:1, 10-500:1, 20-500:1,30-500:1, 40-500:1, 50-500:1, 60-500:1, 70-500:1, 80-500:1, 90-500:1,100-500:1, 150-500:1, 200-500:1, 250-500:1, 300-500:1, 350-500:1,400-500:1.

In one embodiment, the present disclosure provides a method of preparingsubstrate-free 2D tellurene crystals, wherein the method comprisesreacting sodium tellurite (Na₂TeO₃) and hydrazine (N₂H₄) in an alkalinecondition (pH>7) with the presence of polyvinylpyrrolidone (PVP),wherein the reaction temperature is 120-250° C. In one aspect, thereaction temperature is 150-250° C., 160-250° C., 170-250° C., 180-250°C., 160-225° C., 170-225° C., 180-225° C.

In one embodiment, the present disclosure provides a method of preparingsubstrate-free 2D tellurene crystals, wherein the method comprisesreacting sodium tellurite (Na₂TeO₃) and hydrazine (N₂H₄) in an alkalinecondition (pH>7) with the presence of polyvinylpyrrolidone (PVP),wherein the reaction time is 1-100 hours, 1-75 hours, 1-50 hours, 5-100hours, 5-75 hours, 5-50 hours, 10-100 hours, 10-75 hours, 10-50 hours.

In one embodiment, the present disclosure provides a method of preparingsubstrate-free 2D tellurene crystals, wherein the method comprisesreacting sodium tellurite (Na₂TeO₃) and hydrazine (N₂H₄) in an alkalinecondition (pH>7) with the presence of polyvinylpyrrolidone (PVP) toprovide 2D tellurene, adding acetone to the as-synthesized 2D tellurenesolution to provide 2D tellurene crystals with thickness between 1-10nm.

Experiments

Synthesis of 2D Tellurene Crystals

In a typical procedure, analytical grade Na₂TeO₃ and a certain amount ofpoly(-vinyl pyrrolidone) (PVP, M.W.=58000) was put into double distilledwater at room temperature under magnetic stirring to form a homogeneoussolution. PVP at different molecular weight range such as 8000-1,300,000may be used. The resulting solution was poured into a Teflon-linedstainless steel autoclave, which was then filled with an aqueous ammoniasolution and hydrazine hydrate. The autoclave was sealed, and maintainedat the reaction temperature for designed time. Then the autoclave wascooled to room temperature naturally. The resulting silver-gray, solidproducts were precipitated by centrifuge and washed with distilledwater.

Langmuir-Blodgett (LB) Transfer of Tellurene

The hydrophilic 2D Te nanoflake monolayers can be transferred to varioussubstrates by the Langmuir-Blodgett (LB) technique. The washednanoflakes were suspended in a mixture solvent made ofN,N-dimethylformamide (DMF) and CHCl3. Then, the mixture solvent wasdropped into the deionized water. Too much DMF will result in thefalling of 2D Te in the water. It is difficult to mix the DMF, CHCl3 and2D Te when CHCl3 is too much. After the evaporation of the solvent, amonolayer assembly of 2D Te flakes was observed at the air/waterinterface. Then we can transfer the monolayer assembly of 2D Te onto thesubstrates.

First-Principles Calculations

Density Functional Theory calculations were performed using the ViennaAb-initio Simulation Package (VASP)1 with projector augmented wave (PAW)pseudopotentials2. The value 500 eV was used for the plane-wave cutoff,5×5×1 Monkhorst-Pack sampling, and fully relaxed the systems until thefinal force on each atom was less than 0.01 eV/Å. The PBEexchange-correlation functional is used for relaxation of the system,and the HSE functional is employed to calculate the band gaps and theband edge levels. While for bilayer and thicker Te, the structure issimilar to that of bulk Te. The calculations show a lattice parameter of4.5 Å and 6.0 Å for multilayers, in agreement with experiments. Theadsorption of O on bilayer Te and P is modeled by using 4×3 cell.

Structural Characterization

The morphology of the ultrathin tellurene crystals was identified byoptical microscopy (Olympus BX-60). The thickness was determined by AFM(Keysight 5500). High-resolution STEM/TEM imaging and SAED has beenperformed using a probe-corrected JEM-ARM 200F (JEOL USA, Inc.) operatedat 200 kV and EDS has been collected by an X-MaxN100TLE detector (OxfordInstruments). In HAADF-STEM mode, the convergence semi-angle of electronprobe is 24 mrad, and the collection angle for ADF detector was set to90-370 mrad.

Determination of Tellurene Productivity

To quantify the ratio of 2D tellurene flake in the products, all theproducts in the same process were measured as follows: the freshlyprepared 2D tellurene solution was centrifuged at 5000 rpm for 5 minutesafter adding acetone and washed with alcohol and double distilled watertwice. Then the 2D tellurene flakes were dispersed into double distilledwater. After that, the dispersed solution was dropped onto the 1×1 cm²SiO2/Si substrate. After the water evaporation, the optical microscopewas used to record several images randomly covering the 5×5 mm² area. Inthe end, the areas covered by 2D tellurene was analyze by ImageJ, apublic domain, Java-based image processing program developed at theNational Institutes of Health. In the present disclosure, theproductivity was defined as the ratio of the 2D tellurene area in theentire image.

Solvent-Assisted Post-Growth Thinning Process

The as-synthesized 2D tellurene solution (1 mL) was mixed with acetone(3 mL) at the room temperature. After a certain time (e.g. 6 hours), thethin 2D tellurene can be obtained by centrifuge at 5000 rpm for 5minutes. After doing the LB process, the 2D tellurene can be transferredonto the substrate.

Device Fabrication and Transport Measurement

Upon transferring the tellurene flakes onto 300 nm SiO2/Si substrates,source and drain regions were patterned by electron beam lithographywith 2 μm of channel length. The 50/50 nm Pd/Au was chosen as a metalcontact since Pd has relatively high work function thus benefits thep-type transistors by reducing Schottky contact resistance. Thetransport measurements were performed using Keithley 4200A semiconductorcharacterization system. By plugging numbers into the formula:μ_(FE)=g_(m)L_(W)C_(ox)V_(ds)/, where g_(m), and C_(ox) aretransconductance, channel length, channel width and gate oxidecapacitance, the field-effect mobilities for the tellurene transistorscan be derived.

Raman Spectra

Angle-resolved Raman Spectra were measured at room temperature. Thecrystal symmetry of Te renders one A1 mode, one A2 mode(Raman-inactive), and two doublet E modes at Γ point of Brillouin zone.Raman signal was excited by 633 nm He—Ne laser. Three Raman active modeslocating at 92 cm-1 (E-mode), 121 cm-1 (A1 mode) and 141 cm-1 (E-mode)respectively were identified. The incident light comes in along [−1010]direction which is perpendicular to the Te flake surface and waspolarized into [0001] direction, which is parallel to spiral atom chainsand we denote this configuration as 0°. Angle-resolved Raman peakintensity change was observed. The peak intensities of different modeswere extracted by fitting with Gauss function and plotted them intopolar figures. The angle-resolved Raman results confirm that the helicalTe atom chain is indeed along the long axis of the Te flake.

XRD Patterns of the Crystals

The XRD patterns of the crystals are obtained on a D8 Advance X-raypowder diffractometer, equipped with a CuKa source (λ=1.54056 A) and aVantec detector, operating at 40 kV and 40 mA. Each sample is scannedbetween 20° and 65° in 2θ, with a step size of 0.0057° in 2θ and a scanrate of 11.41 seconds/step, and with 2.1 mm divergence and receivingslits and a 0.1 mm detector slit. The washed 2D flakes are directlydeposited on the Si wafer with smooth surface. The crystal formdiffraction patterns are collected at ambient temperature and relativehumidity. The background for the crystal is removed by Jade 6.5 prior topeak picking.

It is well known in the crystallography art that, for any given crystalform, the relative intensities of the diffraction peaks may vary due topreferred orientation resulting from factors such as crystal morphologyand habit. Where the effects of preferred orientation are present, peakintensities are altered, but the characteristic peak positions of the 2DTe are unchanged. Furthermore, it is also well known in thecrystallography art that for any given crystal form the angular peakpositions may vary slightly. For example, peak positions can shift dueto a variation in the temperature or humidity at which a sample isanalyzed, sample displacement, the presence or absence of an internalstandard, or the reaction residue which is not completely removed. Inthe present case, a peak position variability of ±0.1 in 2θ will betaken into account these potential variations without hindering theunequivocal identification of the indicated crystal form.

Conformation of a crystal form may be made based on any uniquecombination of distinguishing peaks (in units of ° 2θ), typically themore prominent peaks. Thus, a prepared sample of 2D Te is characterizedby an XRD pattern using CuKa radiation as having diffraction peaks(2-theta values) as demonstrated in Table 2.

TABLE 2 X-Ray Crystal Diffraction Peaks of 2D Tellurene Angel ° 2θIntensity (%) Crystallographic plane 23.79 100.0 (100) 41.26 29.2 (110)47.79 41.5 (200) 50.41 7.8 (201) 64.43 12.2 (210)

Those skilled in the art will recognize that numerous modifications canbe made to the specific implementations described above. Theimplementations should not be limited to the particular limitationsdescribed. Other implementations may be possible.

The invention claimed is:
 1. A composition comprising substrate-free 2Dtellurene crystals, wherein the 2D tellurene crystals are characterizedby an X-ray diffraction pattern (CuKα radiation, λ=1.54056 A), whereinthe X-ray diffraction pattern consists of five peaks from a range of23.79 (29±0.1°) to 64.43 (29±0.1°) within an intensity range of 7.8% to100%, and wherein the five peaks are ranked in an order of intensityfrom high to low by 23.79 (29±0.1°), 47.79 (29±0.1°), 41.26 (29±0.1°),64.43 (29±0.1°), and 50.41 (29±0.1°).
 2. The composition of claim 1,wherein the 2D tellurene crystals have a lateral length from 0.1-500 μm.3. The composition of claim 2, wherein the 2D tellurene crystals have alateral length from 0.1-100 μm.
 4. The composition of claim 1, whereinthe 2D tellurene crystals have a thickness from 0.5-250 nm.
 5. Thecomposition of claim 4, wherein the 2D tellurene crystals have athickness from 0.5-100 nm.
 6. The composition of claim 1, wherein the 2Dtellurene crystals comprises 2D tellurene single crystals.
 7. Thecomposition of claim 1, wherein the 2D tellurene crystals comprises monolayer 2D tellurene crystals.
 8. The composition of claim 1, wherein the2D tellurene crystals comprises two or more layers of 2D tellurenecrystals.